For many years, there has been increasing interest and a growing demand for equipment and methods that can be used to inspect power generation turbine and generator rotors for material discontinuities and degradation, which can lead to premature and possible catastrophic rotor failure. Through the use of these inspection techniques, rotor life can be extended. The center portion of the steel forging, from which the rotors are made, are the regions most susceptible to rotor discontinuity and other forms of material degradation. In fact, one reason that a central bore hole is machined in most rotors is to provide the capability to remove suspicious material. Moreover, the operating conditions in the region of the central bore holes in these rotors can produce thermal creep, fatigue and thermal embrittlement, particularly, if there are forging discontinuities. It is with these problems in mind that there is a great interest in developing techniques for enabling accurate rotor inspection, especially techniques using non-destructive inspection methods.
Ultrasonic inspection within the bore is a process that has been used since the early to mid-1980's with fairly wide acceptance as a practical high volume inspection technique. Ultrasonic inspection, also known as bore-sonic inspection, uses ultrasonic transducers that are transported through the central bore hole by a convenient method. The transducers generate ultrasonic beams that are directed from the bore surface into the rotor material, producing an ultra sonic wave that penetrates the rotor material. By collecting, processing and observing the reflective characteristics of the wave within the forging, the integrity of the material can be deduced. An entire rotor is inspected by placing the transducers around the rotor circumference and along the length of the bore and directing the ultrasonic beam into the material.
Some traditional methods of ultrasonic rotor bore inspection employ a "contact" probe, which nothing more than a transducer housed on a rigid shoe that slides along the bore surface on a layer of oil. The profile of the shoe is matched to the bore curvature. Though this inspection technique has been widely accepted as a near industry standard, it presents some serious drawbacks. The "contact probe" provides low resolution and sensitivity mainly because the contact shoe has a fixed curvature. Different shoes are therefore required for different bore curvatures, increasing the complexity and cost of the equipment and the time required to perform tests, taking into account the fact that the shoes need to be changed for different rotor shapes.
A second method of ultrasonic rotor bore inspection uses an immersion focus probe. All the transducers are placed within the rotor bore and the bore is then filled with oil. The transducers are moved freely through the oil, with the probe focusing on the bore hole to provide a high resolution and sensitivity reflective signals. The problem with this approach is that it calls for flooding the bore removal of air bubbles and proper positioning of the transducers and sealing the bore once filled with oil. The equipment for this is not portable except perhaps on a truck. In addition, the technique is time consuming and complex, and consequently expensive.